Hermetically sealed devices are used in a variety of applications. Such devices hermetically seal an interior atmosphere within a housing, so that the interior atmosphere is maintained separate from the exterior environment. The interior atmosphere may constitute an inert gas atmosphere, such as helium.
In the manufacturing of data storage devices such as computer hard disc drives with internal inert gas atmospheres, it is often desirable to subject such devices to testing for leakage rates. One possible inert gas for such applications is helium. The upper limit of permissible leakage rate for such devices can be relatively small, such as on the order of 10−8 cc/sec or less. Detection of such small leakage rates can require the use of sensitive gas measuring equipment, such as a mass spectrometer based gas leak detector.
During testing, the inert gas that leaks from the device is normally caused to be collected in a spectrometer tube that is operated at a high vacuum. In practice, the device may be placed in a vacuum tight canister that is connected to the spectrometer, with the canister and spectrometer tube subjected to vacuum evacuation during the test procedure. Since the interior of the device retains helium at approximately 1 atmosphere, subjecting it to a high vacuum during the leak test and placing significant mechanical loads on the device can provide a baseline measure of seal performance.
Testing loads under such conditions are different from, and larger than, those imposed on the device during actual use. Furthermore, it is possible for the testing loads to damage the device, possibly causing temporary, misleading changes in the leak rate.
Attempts have been made to design testing equipment for helium leakage in which the devices are subjected to ambient atmospheric pressure environment. The device is positioned in an airtight inner canister, the walls of which are made of a polymer that is permeable to the helium contained inside the device housing. The inner canister is then placed in another canister that is subjected to a high vacuum, and the leak test is conducted using a gas detector instrument. Helium leaking from the device gradually raises the helium concentration in the airtight inner canister, and the helium gradually permeates through the polymer walls into the evacuated leak detector. In time, the helium concentration in the inner canister and the permeation rate through the container walls, will reach a steady state condition. At steady state, the helium permeation rate through the container walls is equal to the device leakage rate, and the test is concluded.
This approach has major drawbacks. The time required to reach steady state conditions can take an extremely long time, often measured in weeks or months. Also, the helium permeable airtight inner canister must be mechanically strong as it is exposed to a 1 atmosphere pressure differential. Generally, this means that the inner canister may need to be cylindrically or spherically shaped, and have relatively thick walls, typically several millimeters. Further, there is a relatively large volume of air surrounding the device, and since it is difficult for helium to permeate through the thick canister wall, it takes an excessively long time for sufficient helium to leak from the device into the relatively large air volume about the device and then permeate through the thick wall of the outer canister.
There is a need for an inert gas leak tester that does not require that a hermetically sealed electronic device be subjected to a high vacuum, and which is capable of detecting extremely small leakage rates. Further, the leak tester would provide the testing results within the short time requirements of production manufacturing.